Project Euler is a site that has math problems that can be solved with the assistance of a computer program (solving the problems without programming would take an unreasonable amount of time). The problems are fun and they are a good way to learn a programming language and some interesting math.
A few years ago I solved the first 77 problems using Common Lisp.
I intend to eventually continue solving more, maybe switching from Lisp to Matlab, Mathematica, or some other language.
The site requests that users do not share solutions, which I have no intention of doing. However, I did accumulate some helper functions that I wrote while solving the problems, and I think they could be helpful for people getting started on Project Euler using Common Lisp.
Also, check out Solving Project Euler Problems for some additional ideas.